Typically, ball bearings include an inner race, an outer race, a plurality of balls disposed between the inner race and the outer race and circumferentially spaced apart from each other, and a retainer retaining the balls. While metal retainers were used in many of older ball bearings for use in motors because metal retainers are high in rigidity, synthetic resin retainers are increasingly used in recent ball bearings because synthetic resin retainers are quieter and lighter in weight than metal retainers.
A synthetic resin retainers are disclosed, e.g., in JP Patent Publication 2006-226448A and JP Patent Publication 2008-064157A. The synthetic resin retainer disclosed in either of JP Patent Publication 2006-226448A or JP Patent Publication 2008-064157A comprises two axially opposed annular members made of synthetic resin, and each formed with a plurality of circumferentially spaced apart pockets in which the balls are received.
The two annular members of the synthetic resin retainer disclosed in JP Patent Publication 2006-226448A have axially concave arc-shaped pocket wall portions defining the inner surfaces of the pockets in which the balls are received, and a flat plate-shaped coupling plate portions coupling together the adjacent pocket wall portions. The two annular members are ordinarily formed by injection-molding a material comprising a thermoplastic resin such as polyamide to which is added 20-30% by weight of a reinforcing fiber material such as glass fibers.
The axial thicknesses of the coupling plate portions of this type of conventional synthetic resin retainers are about 32-33% of the diameters of the balls, while the axial thicknesses of the pocket wall portions are about 12-15% of the diameters of the balls.
On the other hand, in view of the fact that synthetic resin retainers are lower in strength than metal retainers, synthetic resin retainers are proposed of which the two annular members have increased thicknesses. For example, the synthetic resin retainer disclosed in JP Patent Publication 2008-064157A includes reinforcing walls integral with the coupling plate portions and coupling together the adjacent pocket wall portions in order to increase the strength of the synthetic resin retainer. The reinforcing walls increase the substantive axial thicknesses of the annular members, thus increasing the strength of the synthetic resin retainer.
In designing a synthetic resin retainer comprising the above-described two annular members, the inventor of the present application tried to find an optimal shape of the retainer in preventing damage to the retainer due to stress generated while the bearing is rotating. For this purpose, the inventor conducted a finite element method (hereinafter referred to “FEM”) analysis for annular members each having pockets wall portions and coupling plate portions of which the axial thicknesses are different from the axial thicknesses of those of the other annular members, and studied the results of analysis.
The results of this study revealed that, while it had been considered that the two annular members have to have large thicknesses for increased strength of the retainer, by reducing the thicknesses of the two annular members, stress concentration becomes less likely to occur, so that it is possible to effectively prevent damage to the retainer due to stress generated while the bearing is rotating.
This is presumably for the following reasons. That is, while a bearing is rotating, circumferential pushing forces act between the balls and the retainer due to the balls moving faster or slower than the retainer. If the two annular members have large thicknesses as in the conventional retainers, stress due to the circumferential pushing forces tends to concentrate on portions of the retainer which are shaped such that they are relatively easily deformable (e.g., bent portions of the retainer where the pocket wall portions are connected to the coupling plate portions, and portions of the pocket wall portions where the pockets are the deepest). On the other hand, by reducing the thicknesses of the two annular members, when circumferential pushing forces act on the retainer due to the balls moving faster or slower than the retainer, stress due to the circumferential pushing forces is dispersed throughout the retainer and is less likely to concentrate on limited portions of the retainer, thus reducing the possibility of damage to the retainer.